Formula to return left 6 chartacters if the first characters are numbers and show blank if first characters are text in cell

I am looking for a formula to return the first 6 numbers if the first characters in the cell are numbers and if the characters are text to show a blank cell.

Here is some of the data I am working with.

-DC DONE ON INV 10038 POSTED JULY

-399832 - Credit against CA00662835

-Credit against CA00637779 - PO # 397811 L1 Q30

-415862 - Credit against CA00662378

-429002 L1 Q600

Any help is very much appreciated!

Thanks, Penny